1. Field of the Invention
The present invention relates to alignment of optical elements, particularly in an optical lithography system.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Zoom assemblies having movable optical elements are often used in these lithography systems. In the typical zoom assembly, two or more lenses are driven by motors and/or actuators on a screw drive or other arrangement. The lenses are attached to the motor drive on a lens mount assembly. The lenses are translated along the optical axis such that the distance between the lenses can be manipulated according to the desired zoom characteristics of the lithography platform. This implementation supports continuous zoom functionality. An example optical zoom assembly for a mask-based lithography system is described in U.S. Pat. No. 6,900,946, which is hereby incorporated by reference in its entirety.
In the current mask-based systems, when the zoom setting needs to be changed on the system, the lens positions are translated along the optical axis (e.g., Z). This changes the position of each lens relative to each other. In current zoom design, the lens slide or screw drive arrangement provides a fixed or static reference for the off-axis lens position. During translation, the off-axis position (e.g., X, Y, Rz) of the individual lens elements is subject to changes due to imperfections in travel flatness of the lens slide or screw drive. An example off-axis position change range for the X and Y axes is approximately 3 to 50 microns, with an example off-axis position range for Rz being approximately +/−12 arc-seconds.
For current mask-based systems, these relatively minor off-axis motions are easily accounted for in the error budget for the system. However, for optical maskless lithography systems, the projection optic magnification is approximately 100 times greater than for the mask-based system. Thus, these minor translations of the lens position in the illuminator are intolerable for this system. What is needed is a system for reducing the off-axis position motion to a value that may be tolerated by an optical maskless lithography system.